The present invention relates in general to thermoelectric energy conversion cold end hardware, and more particularly to heat rejection systems thermal connection to thermoelectric conversiondevices.
As is well known, thermoelectricity functions in a manner similar to the distilling of water. Thus, if one junction of a semiconductive thermocouple is heated, electrons are "evaporated" or raised from the valence band to the conduction band. For each evaporated electron, a vacancy or hole is left behind and both the electron and the hole travel to the cold end of the thermocouple through the semiconductors and are reunited at the cold end junction. As the holes and electrons are reunited, a flow of positive current is produced at the cold end junction.
At the cold end of the thermocouple elements, helical compression springs are used to hold these elements in longitudinal compression against a hot plate. Efficient heat transfer to the heat rejection system is one of the requirements for efficient operation of a thermoelectric device and efficient heat transfer requires a good thermal path between the cold junction and the heat rejection system. As the amount of heat rejected at the cold end of the thermalcouple directly affects the efficiency of the thermoelectric generator, such efficiency can be increased by increasing the efficiency with which heat is transferred to the heat rejection system. Springs are often used to insure good thermal (and in some cases electrical) contact between the cold and hot ends of the thermocouples and the heat rejection and heat source systems. As springs present a small cross-sectional area with a long path and are in nonbonded pressure contact at both ends and must usually be made of low conductivity metals they are not adequate alone for efficient heat transfer but must be augmented in some way.
Springs are a desirable way to achieve the above loading requirements as they also will function to accommodate misalignment between the cold junction of the thermocouple and the heat rejection system. Furthermore, they will compensate for the unavoidable manufacturing tolerances which accrue during the fabrication of a generator. Thus helical compression springs are one useful way of applying the required T/E element end loads in thermoelectric generators.
Some attempts have been made to overcome the drawback of the limited heat transfer path of springs. These attempts are generally directed to providing heat transfer paths between a thermoelectric cold end junction and a heat rejection system, which paths are in addition to that path provided by a compression spring. One such device is shown in Spira et al. U.S. Pat. No. 3,266,944, wherein a flexible braid or conductor is positioned axially within the spring and maintains positive electrical and heat exchange relation between spring seats positioned on opposite ends of cold end compression springs. However, the cross-sectional area of the conductor shown in the Spira device is restricted by the interior diameter of the spring as well as the fact that it is formed of many fine wires and thus the heat transfer path between the thermoelectric generator cold end junction and the heat rejection system is limited. Other known devices generally circulate a coolant between the generator cold end junction and the heat rejection system in order to increase heat transfer therebetween, or require volumes and densities of materials greatly in excess of those necessary for the flexible sleeve to achieve similar results.
The present invention bypasses the poor thermal path provided by the loading spring and provides a highly conductive one between the thermoelectric element cold junction and the heat rejection system. It is comprised of a flexible sleeve device positioned around the thermoelectric loading spring to provide the good heat transfer path. The flexible sleeve is bonded to the heat rejection system, electrically insulated from the thermoelectric element cold end conductor and is longitudinally collapsible in such a manner as to apply only a negligible load to the thermoelectric element and thus not interfere with the operation of the loading spring, or add to or subtract from its loading force on the element.